The present invention relates in particular to a transmission system comprising a transmission shaft presenting two longitudinal ends, in which a connection member, such as a deformable coupling member, is fitted to each end of the shaft, the two connection members connecting the shaft respectively to two rotary members of the rotorcraft such as a main gearbox (MGB) at one end and a tail rotor gearbox (TRG) at the other end, thus enabling the shaft to drive a rear rotor, or tail rotor, of the rotorcraft from the MGB via the TRG.
Patents FR 2 817 234 and U.S. Pat. No. 6,680,554 describe such a power transmission system between two gearboxes of a helicopter, the system including a (hollow) tube acting as a transmission shaft.
The shaft is said to be “supercritical”, which means that the frequency corresponding to the speed of rotation of the shaft (in its normal operating range) is higher than the lowest resonant (natural) frequency of the shaft.
An active magnetic damper is placed between the two gearboxes; the damper is controlled by a computer making use of signals delivered by a sensor that is responsive to the lateral (radial) position of the shaft, and including a proportional, integral, and differential (PID) regulator for the purpose of damping the vibrations of the shaft as it passes through resonant frequencies.
The magnetic damper corresponds to an active radial magnetic bearing that damps the vibrations of the shaft, it presents radial stiffness that is low enough to avoid shifting the resonant frequencies of the shaft by more than 3%, and it presents clearance of 3 millimeters (mm) to 4 mm relative to the shaft.
As a result in particular of the complexity and the critical nature of the hardware and software components of an active magnetic bearing, implementing them on board a rotorcraft suffers from the high development costs that are necessary in order to enable them to be qualified or certified.
Patents EP 1 068 665 and U.S. Pat. No. 6,268,676 describe a system for supporting a shaft that includes a passive radial magnetic bearing in which the magnetized stator portion (i.e. the non-rotary portion) is connected to a support via four elastomer dampers.
The mass and the stability of such a system make it difficult to use for supporting a rotorcraft transmission shaft.